This invention relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system with ultrasound autocapture capability for determining whether a stimulation has evoked a desired response from the heart.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is providing therapy at appropriate energy levels. In pacers, for example, pacing stimulations must have sufficient energy to capture the heart, that is, to initiate a resulting heart contraction. Delivering too much energy, however, will shorten the life of the battery powered implantable device. This, in turn, results in performing an earlier surgical explantation and replacement procedure, with its attendant risks and costs, both for the procedure and for the replacement device. Thus, there is a need to determine whether a cardiac rhythm management system is providing therapy at appropriate energy levels.
This document describes, among other things, a cardiac rhythm management system with ultrasound autocapture capability for determining whether a stimulation has evoked a desired response from the heart, and for adjusting an energy of the stimulation based on the observed response from the heart. An autocapture determination circuit determines whether motion of the heart chamber indicates a contraction in response to the stimulation, and adjusts the stimulation energy to provide only that energy which is needed to ensure reliable capture. This saves energy, prolonging the life of the implanted device, minimizing the risk and expense to patient associated with early explantation and replacement of the implanted device.
In one embodiment, the cardiac rhythm management system includes a lead. The lead includes a distal end and a proximal end. The distal end of the lead is adapted for being disposed in or about a heart. The distal end of the lead includes a first ultrasonic element. An electronics unit is coupled to the proximal end of the lead. The electronics unit includes an ultrasound driving circuit and a signal processing circuit that includes an autocapture determination circuit. In a first further embodiment, a second ultrasonic element is adapted for being disposed in a heart chamber different from the first ultrasonic element. In a second further embodiment, a second ultrasonic element is in a case carrying the electronics unit.
This document also discloses using the ultrasound for applications other than the ultrasound capability, including but not limited to: (1) determining the strength of heart contractions, such as from the slope of a signal transduced from the ultrasound, (2) determining dissociation between electrical and mechanical heart activity, (3) determining the volume of the heart, such as at different times during the cardiac cycle, (4) determining the origin of sensed intrinsic electrical heart activity signals based at least in part on information obtained from the ultrasound, (5) recognizing particular arrhythmias based at least in part on information obtained from the ultrasound, (6) delivering ultrasound for disrupting cell membranes for lowering stimulation thresholds, (7) using ultrasound to control the delivery of a steroid, and (8) obtaining blood flow information based at least in part on information obtained from the ultrasound. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.